The operation of an internal combustion engine, such as, for example, a diesel, gasoline, or natural gas engine, may cause the generation of undesirable emissions. These emissions, which may include particulates and nitrous oxide (NOx), are generated when fuel is combusted in a combustion chamber of the engine. An exhaust stroke of an engine piston forces exhaust gas, which may include these emissions from the engine. If no emission reduction measures are in place, these emissions will be exhausted to the environment.
Research is currently being directed towards decreasing the amount of certain emissions that are exhausted to the environment during the operation of an engine. It is expected that improved engine design and improved control over engine operation may lead to a reduction in the generation of these emissions. Many different approaches, such as, for example, engine gas recirculation and aftertreatments, have been found to reduce the amount of emissions generated during the operation of an engine. Unfortunately, the implementation of these emission reduction approaches typically results in a decrease in the overall efficiency of the engine.
Additional efforts are being focused on improving engine efficiency to compensate for the efficiency loss due to the emission reduction systems. One such approach to improving the engine efficiency involves adjusting the actuation timing of the engine valves. For example, the actuation timing of the intake and exhaust valves may be modified to implement a variation on the typical diesel or Otto cycle known as the Miller cycle. In a “late intake” type Miller cycle, the intake valves of the engine are held open during a portion of the compression stroke of the piston.
The engine valves in an internal combustion engine are typically driven by a cam arrangement that is operatively connected to the crankshaft of the engine. The rotation of the crankshaft results in a corresponding rotation of a cam that drives one or more cam followers. The movement of the cam followers results in the actuation of the engine valves. The shape of the cam governs the timing and duration of the valve actuation. As described in U.S. Pat. No. 6,237,551 to Macor et al., issued on May 29, 2001, a “late intake” Miller cycle may be implemented in such a cam arrangement by modifying the shape of the cam to overlap the actuation of the intake valve with the start of the compression stroke of the piston.
However, a late intake Miller cycle may be undesirable under certain operating conditions. For example, a diesel engine operating on a late intake Miller cycle will be difficult to start when the engine is cold. This difficulty arises because diesel fuel combustion is achieved when an air and fuel mixture is pressurized to a certain level. Implementation of the late intake Miller cycle reduces the amount of air and the amount of compression within each combustion chamber. The reduced compression results in reduced pressure, which in turn results in reduced temperature of the engine and in a lower maximum pressure level of the air and fuel mixture. Thus, achieving combustion in a cold engine operating on a late intake Miller cycle may prove difficult.
As noted above, the actuation timing of a valve system driven by a cam arrangement is determined by the shape of the driving cam. Because the shape of the cam is fixed, this arrangement is inflexible and may not be changed during the operation of the engine. In other words, a conventional cam driven valve actuation system may not be modified to account for different operating conditions of the engine.
As a further example, U.S. Pat. Nos. 7,004,122 and 7,258,088 describe an intake valve actuation system including a control valve for managing a flow of fluid to a fluid actuator. In addition to the control valve, U.S. Pat. Nos. 7,004,122 and 7,258,088 describe a configuration of parallel fluid passages between the control valve and the fluid actuator. In particular, a directional control valve and a check valve are configured along respective fluid passages of the parallel fluid passages to effect further fluid control to and from the fluid actuator. However, improvements in valve actuation systems are needed, for example, to lower the compression ratio and peak cylinder pressures in a large dual fuel diesel and natural gas engine to allow for maximum gas substitution while being emissions compliant. Accordingly, these and other shortcomings of the prior art are addressed by the present disclosure.